1. Field of the Invention
The present invention relates to an electrophotographic photoconductor. More particularly, the present invention relates to a photoconductor that exhibits improved charge retention rate in the dark and exhibits additional desirable charging characteristics including stability of potential in repeated use and diminished image defects. The invention may be favorably applied to an electrophotographic photoconductor that is used in an electrophotographic apparatus employing discharged-area development process system.
2. Description of the Related Art
Many business machines employ a type of image formation based on electrophotographic technology. These business machines include copiers, printers, plotters, and composite digital imaging systems which combine functions of several such machines.
Recently, this imaging technology has been widely applied to small-sized printers and facsimile machines. Since Carlson's invention (U.S. Pat. No. 2,297,691), a wide variety of photoconductors have been developed for electrophotographic apparati, in which photoconductors employing organic material have been commonly used.
In known function-separated organic photoconductors, laminates, on a conductive substrate include an undercoat layer of an anodic oxide film or a resin film, a charge generation layer containing photoconductive organic pigment such as titanylphthalocyanine or azo pigment, a charge transport layer containing a molecule with a partial structure involved in hopping conduction of electric charges. Examples of such molecules including amine and hydrazone that is combined with a π-electron-conjugated system, and a protective layer.
Known photoconductors also include single-layer photoconductors that comprise a photosensitive layer, functioning as both charge generator and charge transporter, laminated on an undercoat layer, and a protective layer if necessary.
Electrophotographic apparatuses in recent years commonly employ a discharged-area development process, in which digital signals of pictures and characters are transformed to optical signals using a light source of a semiconductor laser or a light emitting diode with a wave length of from 450 nm to 780 nm. Here, the optical signals illuminate a charged photoconductor to form a latent image on the photoconductor surface, and the latent image is then visualized using toner powder.
Organic pigments composed of a phthalocyanine have been extensively studied recently as a material for use in a photosensitive layer, because that material exhibits large absorption coefficient in the above-mentioned wavelength range of a semiconductor laser, and since as compared with other charge generation substances, the material has excellent charge generating capability.
Photoconductors are known that use, in addition to the phthalocyanine having a central metal of titanium, a phthalocyanine having a central metal of copper, aluminum, indium, vanadium, as disclosed in Japanese Unexamined Patent App. Pub. Nos. S53-89433 and S57-148745, and U.S. Pat. Nos. 3,816,118 and 3,825,422.
In recent years, the phthalocyanine having a central metal of titanium is used by preference because the material exhibits large absorption coefficient and high sensitivity in the wavelength range of a semiconductor laser.
Unlike a charged-area development process, in the above-mentioned discharged-area development process, a dark potential corresponds to a white portion and a bright potential corresponds to a black portion of an image. Therefore, if the photosensitive layer laminated on the conductive substrate includes an organic pigment particle for charge generation with extremely large size, an image defect such as a black spot or a fog in a white matrix may unfortunately be generated.
This kind of undesirable defect is believed to be caused by minute leakage of electric charges from the conductive substrate through the large-sized pigment particle to the surface of the photosensitive layer; this leakage in turn causing local decrease of electric potential.
An electrophotographic apparatus that employs both discharged-area development and contact charging, in particular, in which the photoconductor directly contacts with a charging member, is unfortunately liable to raise this image defect problem. In order to mitigate this problem, it is known to be effective to form the charge generation layer by means of evaporation method. However, as a further concern, the evaporation method undesirably needs to employ a batch production system, must use expensive vacuum equipment, and requires exposure to a solvent atmosphere after deposition in order to transform to a proper crystal form. Consequently, a method which employs both discharged-area development and contact charging results in high manufacturing costs, which opposes the recent trend of cost reduction and causes serious economic concerns in business.
Additionally, there often arose a problem wherein a photosensitive layer formed from certain lot of coating liquid for a charge generation layer containing titanylphthalocyanine pigment did not exhibit enough charge retention rate in the dark and showed deteriorated charging characteristics after repeated use, although high sensitivity was temporarily achieved.
In Japanese Unexamined Pat. App. Pub. No. H1-97965 an electrophotographic photoconductor exhibiting high sensitivity and excellent characteristics after repeated use was obtained by controlling the crystallite diameter of the diazo compound to be not less than 11 nm. This invention in the publication is limited to the crystallite diameter of the diazo compound. The inventor of the present invention noted the disclosure about the crystallite diameter in the publication when he was investigating the above-described problem of the lot-to-lot variation in the characteristic of the coating liquid containing titanylphthalocyanine pigment.
In Japanese Unexamined Pat. App. Pub. No. 2000-147811 discloses a memory phenomenon wherein, in a printed image, point defects such as black spots or white spots were prevented by controlling the particle size of a metal-free phthalocyanine or a phthalocyanine having a central metal of titanium as a charge generation substance in a single-layer type photosensitive layer to distribute in the range from 0.3 μm to 2 μm.
In Japanese Unexamined Pat. App. Pub. Nos. H4-198367 and H4-95964 it is disclosed that electrophotographic characteristics are improved by specifying the relation between the crystal form and the specific surface area and the relation between the crystal form and the particle diameter of a titanylphthalocyanine.
Unfortunately, the above publications do not disclose the improvement of electrical characteristics in a photoconductor by controlling both a crystallite diameter and a primary particle diameter of the titanylphthalocyanine pigment as disclosed in the present invention.